Porous materials find application in modern technologies including thermal insulation, gas storage, active membranes, fuel cells, solar cells, batteries, supercapacitors, drug delivery, bio-implants, sensors, photonics, water purification, and the like, owing to their high surface area and porosity. Examples include porous metal oxides, organic polymers, and carbon, as well as composites thereof.
Nanoporous materials, for example, having a pore distribution in the nanometer range, can be synthesized by a template method or a solvent removal method.
In the template method, sacrificial templates can be mixed into a liquid body of the precursor that polymerizes to form a solid. Then, the templates can be removed to leave pore space in the solid body. The templates can be organic polymers, self-assemblies of surfactants, or other nanoparticulate matter. In some cases, the sacrificial templates can be a pre-formed bulk porous solid. In such cases, the porous body can be infiltrated by the liquid precursor by soaking the body in the liquid. For example, porous carbon can be produced by infiltrating chromatography-grade porous silica with a solution of phenol, formaldehyde, and tetraethylammonium hydroxide; polymerizing the phenol and formaldehyde in the pores of silica; pyrolyzing and carbonizing the polymer; and etching the silica out from the material, as described in U.S. Pat. No. 4,263,268, entitled “Preparation of Porous Carbon,” to Knox et al., which is incorporated herein by reference. The tetraethylammonium hydroxide acts as a catalyst for the polymerization of phenol and formaldehyde.
The solvent removal method can start with formation of a wet gel. In a wet gel, the solid component is formed by “sol” nanoparticles that are held together loosely but continuously throughout the entire body of the gel. The solvent can be removed, for example, by drying, to leave a porous material. The porosities and the pore morphologies of the resulting materials can be affected by the liquid removal methods such as heating, ambient drying, supercritical drying, cryogenic drying, and the like. The gels can be made of various materials including oxygen-containing metal compounds and organic polymers. Pyrolytic polymer gels can be carbonized to form porous carbon.
In some cases, the wet gel can be an “interpenetrating” inorganic-organic composite gel. The subsequent pore liquid removal can provide a porous composite material made of different compounds. In certain cases, interpenetrating inorganic-organic composite gels are prepared by starting with a wet gel that has only one solid network component, inorganic or organic. Through a solvent exchange process, the pore liquid of the wet gel is replaced by another liquid that includes the precursors for the other network component. This method can be time-consuming and can require excessive precursor chemicals.
U.S. Patent Application Publication No. 2010/052276 entitled “Fabricating porous materials using thixotropic gels,” to D.-K. Seo and A. Volosin, which is incorporated by reference herein, shows that interpenetrating inorganic-organic composite gels can be prepared by first preparing an inorganic thixotropic gel such as alumina gel. A solution of organic polymer precursors is added to the thixotropic gel, while the gel is sheared and thus becomes liquefied. Upon removing the shear, the material gels again with the organic polymer precursors in the pore liquid.
In other cases, inorganic gel precursors and organic polymer gel precursors are premixed first in a solvent and then an interpenetrating inorganic-organic gel is formed by promoting “simultaneous” gelation of both inorganic gel precursors and organic polymer gel precursors in the solution. In U.S. Pat. No. 5,254,638, entitled “Composite materials of interpenetrating inorganic and organic polymer networks,” to B. M. Novak et al., which is incorporated herein by reference, interpenetrating networks of silica (or titania) and polymerized alcohols are reported to form through hydrolysis of alkoxides of silicon (or titanium), polycondensation of the hydrolyzed alkoxides and polymerization of alcohols. The hydrolysis, polycondensation, and polymerization occur “concurrently” in a solution and are catalyzed by a common acid that is added to the solution together with the precursors. The composite gels are then supercritically dried to form composite aerogels. It is noted that these co-gelation methods are different from other procedures for porous inorganic-organic composites that are based on “copolymerization” between inorganic and organic gel precursors. [See for example, C. Moreno-Castilla and F. J. Maldonado-Hódar, “Synthesis and surface characteristics of silica- and alumina-carbon composite xerogels” Phys. Chem. Chem. Phys. 2000, 2, 4818, which is incorporated herein by reference].
Such co-gelation methods have been reported for metal oxides other than silica or titania in preparation of porous interpenetrating metal oxide-polymer composites. Polymerization of certain organic gel precursors such as resorcinol-formaldehyde pair can be catalyzed by either an acid or a base. Some inorganic salts are acidic in water and thus can act as both an acid catalyst for the organic polymerization and a source for metal oxide in the final product. For example, N. Leventis et al. has shown that CuO/resorcinol-formaldehyde gels can be prepared by two sol-gel processes running “concurrently” in a mixture of CuCl2.xH2O, resorcinol, formaldehyde, and epichlorohydrin in an H2O/DMSO solvent at 80° C. for 4 hours. [See N. Leventis et al., “One-pot synthesis of interpenetrating inorganic/organic networks of CuO/resorcinol-formaldehyde aerogels: nanostructured energetic materials” J. Am. Chem. Soc. 2009, 131, 4576, which is incorporated by reference herein.] In their work, the CuO precursor, CuCl2.xH2O, was shown to be acidic (pH˜3) in the H2O/DMSO solvent, and the acid catalyzed the polymerization of the resorcinol and formaldehyde. The CuO/resorcinol-formaldehyde gels were supercritically dried to provide CuO/resorcinol-formaldehyde composite aerogels.
In some cases, it is unclear whether or not both the inorganic and the organic components form network structures throughout the solution in “concurrent gelation” methods. For example, the work by R. Vendamme et al. on preparation of zirconia-polymer composite membranes raises the problem and indicates that their porous membranes may have “partial” networks of zirconia instead of “continuous” networks. [See R. Vendamme et al., “Robust free-standing nanomembranes of organic/inorganic interpenetrating networks” Nature Mater. 2006, 5, 494, which is incorporated herein by reference.]
In some cases, the network structures are used further to fabricate a new porous material by removing either the inorganic or the organic network component. For example, removal of the metal oxide component from a porous interpenetrating metal oxide-carbon composite will provide a porous carbon, and removal of the carbon component from a porous interpenetrating metal oxide-carbon composite will provide a porous metal oxide. If one of the network components is not sufficiently continuous, the porous material from the removal of the other component may not be continuous, either. In addition, a “partial” or incomplete network component can be harder to etch out when it is fully surrounded by the other network component and thus less easily accessed by an etching agent. Furthermore, if the material is used as an electrical conduction medium, the resulting porous material may be less conductive at least in part because of the partially continuous network structure.